As an optical fiber with a low Rayleigh scattering loss and a low transmission loss, silica-based optical fibers each having a core doped with an alkali metal element are known as can be seen from, for example, Japanese Patent Application (translation of PCT application) Laid-Open No. 2005-537210W (Patent Literature 1) and US Patent Application Laid-Open. No. 2006/0130530 (Patent Literature 2). Inclusion of an alkali metal element in a core portion of an optical fiber preform can reduce viscosity of the core portion in drawing the optical fiber preform itself and uniforms a network structure of a silica glass. Therefore, the Rayleigh scattering loss attributable to an ununiform structure can be reduced.
As a method for doping an alkali metal element in a silica glass, a diffusion method is known as can be seen from, for example, Patent Literature 1 and Patent Literature 2. The diffusion method is to diffuse and dope an alkali metal element in an inner surface of a glass pipe by heating the glass pipe by an external heat source or generating plasma in the glass pipe while a raw material vapor such as an alkali metal element or alkali metal salt serving as a raw material is introduced into the glass pipe.
After doping the alkali metal element into a vicinity of the inner surface of the glass pipe in this way, the glass pipe is heated for diameter reduction. After the diameter reduction, the inner surface of the glass pipe is etched by a certain thickness with an aim to remove a transition metal element such as Ni (nickel) and Fe (iron), added simultaneously in doping an alkali metal element. An alkali metal element is faster in diffusion than the transition metal element. Therefore, even after a transition metal element is removed by etching a glass surface by a certain thickness, it is possible to cause the alkali metal element to remain. After etching, a core rod containing an alkali metal element is manufactured by heating the glass pipe for collapse. It is possible to apply a glass serving as a second core portion on the outer periphery of the core rod containing an alkali metal element, and handle the whole of the core rod and the second core portion as a part to be a core of the optical fiber.
The optical fiber preform is manufactured by providing a cladding portion on the outer periphery of the core portion, and the cladding portion has a refractive index lower than that of the core portion which includes the core rod containing an alkali metal element. Then, by drawing of the optical fiber preform, the optical fiber can be manufactured.